1. Field of the Invention
The present invention relates generally to bicycle drive systems, and more particularly, to a hollow shaft and crank arm assembly comprising a tubular housing surrounding first and second shaft members that are matingly engaged.
2. Description of the Background Art
Common bicycle drive systems typically comprise a solid cylindrical shaft connected to solid crank arms. However, in today's market, cyclists desire the reduced drag and increased acceleration benefits provided by reduced weight bike parts. Reduction in weight is accomplished using tubular shafts and hollow crank arms instead of solid shafts and crank arms, respectively. To further reduce the weight, lighter, composite materials have been substituted for the more commonly used heavy metals.
For example, U.S. Pat. No. 2,350,468 describes a crankshaft assembly utilizing hollow crank arms attached to a solid shaft housed in a metallic sleeve. The weight of the solid shaft is not addressed. The solid metal shaft adds undesirable weight to the crankshaft assembly, and the undesirable weight adversely affects bicycle acceleration.
U.S. Pat. No. 4,704,919 describes a two piece crankshaft for a bicycle; the crankshaft comprises a first fixed crank arm welded to one end of a tubular spindle with a spline at its other end. A second removable crank arm is welded to an internally splined boss which engages the splined end of the spindle. This configuration does not permit maximum translation of pedal force to rotation force because the spline junction is disposed at the end of the spindle. While the use of a hollow spindle reduces the overall weight of the crankshaft assembly, the spline junction of the second crank arm to the shaft does not provide the assembly with maximum strength because the junction is disposed proximate an end of the removable crank arm, which is an area of the crankshaft assembly that receives the greatest loads. Because the strength of the crankshaft assembly is reduced by the design of the spline junction, the useful life of the crankshaft assembly is limited. Lastly, the splines on the spindle end and second removable crank arm are more expensive to manufacture than a plain cylindrical shaft end, and thus, increase the overall cost of manufacture for the crankshaft assembly.
U.S. Pat. No. 4,811,626 discloses a hollow crank arm comprising three separate inter-assembled segments made of composite material. The use of composite material decreases the overall weight of the crank arm, and concomitantly the weight of the crankshaft assembly. However, the use of composite material is very expensive and greatly increases the cost of manufacture of the part.
As none of the prior art discussed above successfully provides a crankshaft assembly having the lightweight and high strength characteristics desired by cyclists, what is needed is a light weight crankshaft assembly that includes hollow crank arms and a tubular shaft, that has a simple design to reduce manufacturing costs, and that is made of material having a high strength to weight ratio. Further, there is a need for a crankshaft assembly having the junction coupling the right and left crank arms positioned away from the area of greatest load upon the crankshaft assembly to maximize the strength and useful life of the crankshaft assembly.